The present invention relates to an AC power source apparatus for use where an AC power source having an output frequency in a range from a commercial-base frequency of 50 Hz/60 Hz to a low frequency of the order of several kHz is required. The invention can be used, for example, in AC corona generators used in a de-electrification/separation process, which is one of electrographic image forming processes performed in electronic duplicators, laser beam printers or the like; in electric motors; in uninterruptible power supplies; and so on.
As one of AC power supply means in the prior art, a voltage resonance type DC--AC inverter has been disclosed in JP-B-62-16078. FIG. 1 is a circuit diagram of the voltage resonance type DC--AC inverter, and FIG. 2 is a graph showing waveforms at various points depicted in FIG. 1. Referring now to FIG. 1, a DC power source 14 is connected between power supply terminals 12 and 13, one terminal 12 being connected to an intermediate tap of a primary winding of a transformer 1 through a primary winding of an inductance element 5 having a reset winding. A resonance capacitor 11 is connected across the primary winding of the transformer 1 and connected to the other power supply terminal 13 via a common junction point of a pair of switching transistors 2 and 3 and through the collector-emitter circuits of both switching transistors 2 and 3. A pulse width control oscillator 4 generates a pulse signal having a predetermined frequency and a variable pulse width. The pulse signal is fed to the bases of the transistors 2 and 3 to turn on the transistors alternately. One end of a secondary winding of the inductance element 5 is connected to the power supply terminal 12 through a diode 6 arranged in series in the forward direction. The other end of the secondary winding 12 of the inductance element 5 is connected to the power supply terminal 13. The transformer 1 is provided with a gap for adjusting inductance so that a resonance frequency produced by a combination of the transformer 1 and the resonance capacitor 11 coincides with an oscillation frequency of the pulse width control oscillator 4. In the aforementioned construction, a sine-wave AC voltage V.sub.AC can be produced in the secondary winding of the transformer 1 by turning on the transistors 2 and 3 alternately through output pulse signals generated from the pulse width control oscillator 4.
As shown in FIG. 3, the pulse width control oscillator 4 is constituted by an oscillation circuit 15, a pulse width modulation circuit 16 and a distribution circuit 17, the output pulse width of the pulse width modulation circuit 16 being controlled by a control signal 18. When the output pulse signals from the pulse width control oscillator 4, for example, rectangular pulse signals having waveforms (a) and (c) as shown in FIG. 2, are respectively applied to the bases of the transistors 2 and 3, the collector voltages of the transistors 2 and 3 change as shown by waveforms (b) and (d) in FIG. 2. Because the current in the primary winding of the inductance element 5 is cut off while the transistors 2 and 3 are turned off simultaneously, excessive voltages are apt to be induced across the primary winding and at the respective collectors of the transistors 2 and 3. However, a voltage Vid is induced across the secondary winding of the inductance element 5, so that when the voltage Vid exceeds the voltage Vdc of the DC power source 14, the excessive energy of the voltage Vid is regeneratively fed back to the DC power source 14 through the diode 16. Accordingly, the peak values of the collector voltages of the transistors 2 and 3 are limited to predetermined values. Thus, the conventional system is constructed so that the sine-wave output voltage Vac produced across the secondary winding of the transformer 1 can be changed suitably by changing the pulse width of the output signals generated by the pulse width control oscillator 4.
However, the conventional system requires wave shaping of the output voltage by resonance due to the capacitor 11 and the inductance of the transformer 1. Accordingly, the capacitor 11 must have a large capacity. When the capacitor 11 has a sufficiently large capacity to assure stable resonance with a certain degree of Q, the loss due to the resonance current and the DC resistance components of the primary windings of the inductance element 5 and the transformer 1 as well as the dielectric loss of the capacitor 11 become large. These losses cause deterioration of the power supply efficiency. Further, stabilization of resonance frequency and improvement of Q are required for keeping the predetermined output waveform stable. Further, adjustment of the core gap of the transformer 1 and adjustment of the capacitor 11 are required for tuning the oscillation frequency to the resonance frequency. Accordingly, the conventional system is inferior not only in productivity but also in efficiency of the transformer 1 because of existence of the gap thereof. Consequently, the efficiency of power supply is apt to be deteriorated. Further, the output frequency cannot be changed easily, because tuning is required. Because .mu. of the core and the equivalent gap vary with the change of temperature and the passage of time, tuning becomes difficult. Consequently, the conventional system has a disadvantage in that the waveform and amplitude of the output voltage cannot be kept constant. Further, in the case where a time ratio is changed corresponding to the change of the input and load, the peak factor of the output waveform is changed as follows. When the time ratio of the output pulse signal from the pulse-width-modulated oscillator 4 is small as shown in FIG. 4(a), the peak factor is heightened as shown in FIG. 4(b). When the time ratio of the output pulse signal is large as shown in FIG. 4(c), the peak factor is lowered as shown in FIG. 4(d). Consequently, the conventional system has a disadvantage in that the peak factor of the output wave varies widely according to the change in the input and the change in the load.
Further, in general, two types of output waveforms, that is, sine wave and rectangular wave, are used as an AC power supply for electrography. However, the conventional system has an disadvantage in that the rectangular wave cannot be used because the conventional system utilizes resonance. Further, it is difficult to make the switching frequency of the transistor high, because, in general, the switching frequency is a low frequency like the frequency of the output voltage. Accordingly, a large inductance is required. Consequently, the conventional system has a disadvantage in that a small-scale and low-cost system cannot be provided.